The invention concerns a method of manufacturing a seamless self-supporting sheet metal part using the nickel carbonyl vapour deposition process to replace conventional sheet metal parts that are formed from flat blanks and welded or brazed together.
Conventional methods of forming sheet metal parts by various methods are well known in the art generally involving the use of flat sheet metal stock or coils of sheet metal which are flattened prior to cutting into various flat blank shapes.
Finished sheet metal components for various uses are conventionally bent, roll-formed, stamped and formed into shapes that are combined with other shapes and welded or brazed into a final sheet metal assembly.
In the production of aircraft engines in particular, the geometric complexity of sheet metal components and the accuracy required can be very demanding. The efficiency of an aircraft engine may depend heavily on the degree of shape accuracy of the fabrication of various sheet metal components. The surface finish of the components and finish of welds and other surface discontinuities can lead to aerodynamic inefficiencies, stress concentrations or possibly fatigue failure under thermal or dynamic loading.
As a consequence, the complex shapes constructed of sheet metal for aircraft engines involve a high degree of precision, skill, quality control, rework, part rejection and the necessary expenditure of time and money to produce acceptable results.
An example of a complex sheet metal component is an exhaust mixer for an aircraft engine. This component is an annular ring with a pleated or accordion-like skirt which serves to mix the cold engine bypass flow and hot engine exhaust gasses at the tail of the engine. Such complex shapes are generally fabricated by cutting and forming smaller segments in a sheet metal die press then assembling the components together in a complex welding jig. The fitting and assembly of multiple components inherently involves a degree of inaccuracy. As well, the introduction of heat to thin sheet metal components during welding introduces inaccuracies and heat distortion as the adjacent metal expands and contracts. Welding of aerodynamic shapes is less than ideal since the surface of welded areas must be ground, polished or finished to preserve the surface aerodynamic properties of the part. The heat from welding induces residual stresses which are locked into the structure and require heat treating to relieve these built-in stresses, that introduces distortion.
Where the accuracy of forming thin metal components is critical, the part may be machined from a casting or forging. However, machining involves relatively high expenses and labour as well as significant cycle time in manufacture. Accordingly, machining of very thin sheet metal components can be considered as the last resort that is only justified where the high cost of machining is necessary to ensure the accuracy and efficiency of the engine component. In some cases it is not possible to achieve the desired thickness.
U.S. Pat. No. 5,444,912 to Folmer shows a method of forming a flat blank into an exhaust mixer using hydraulic pressure and a complex forming jig. However, due to the complexity of this mechanism and the need to manufacture a separate jig for each different exhaust mixer configuration, this approach is of limited application.
A far greater disadvantage of conventional methods however is that the aerodynamic efficient design of the exhaust mixer and other sheet metal components are severely restricted by the method of manufacture.
The designers of aircraft engines may in theory design shapes for various components, that would result in increases in efficiency or optimize efficiency. However, these innovations are rendered impractical and uneconomical since the complex shapes that result would be prohibitively expensive to manufacture using conventional metal forming methods.
Therefore, the efficiency of an aircraft engine is severely restricted not by lack of technical design expertise but by the manufacturing methods used for economically producing the sheet metal components.
To date, the trade-off between efficiency of design and efficiency of manufacture has favoured manufacturing due to the high cost of producing complex sheet metal shapes.
The inventors have recognized that nickel vapour deposition may be utilized to produce complex sheet metal surfaces despite the relatively high cost involved and the long periods of time required to deposit pure nickel on a mould surface. In addition, the 99.9% pure nickel deposited in this process has limited capacity for heat resistance making it unsuitable for applications where operating temperatures exceed 900xc2x0 F.
Despite these limitations and the generally high temperatures inside aircraft engines, the inventors have recognized that engine exhaust mixers can justify this slow and expensive process being complex sheet metal components exposed to relatively low temperatures of about 600xc2x0 F. Cost/Benefit analysis will also reveal other sheet metal components for various low temperature applications where the invention can be justified within gas turbine engines and elsewhere to replace conventional sheet metal fabrications.
Nickel vapour deposition is commonly utilized for low temperature nickel plating of electrodes or other metal clad components as illustrated in the following U.S. patents: U.S. Pat. No. 4,687,702 to Monsees for applying a metal layer on one surface of a polyamide foam; and U.S. Pat. No. 5,362,580 to Ferrando et al. for a nickel coated lightweight battery electrode.
Another common application of nickel vapour deposition is in the manufacture of nickel plated moulds for plastic injection moulding. Various methods and apparatus for producing nickel plated moulds for plastic injection moulding are described in U.S. Pat. No. 5,591,485 to Weber et al. U.S. Pat, No. 5,470,651 to Milinkovic et al. and U.S. Pat. No. 5,570,160 to Weber et al.
In the prior art, the nickel deposition process is used to apply a thin very accurate plating or coating on a substrate. The nickel coating layer conventionally provides the properties of electrical conductivity or wear resistance as in U.S. Pat. No. 5,934,157 to Kobayashi et al. In the case of manufacturing plastic moulds, nickel deposition is used to provide an accurate mould surface that is rigid, wear resistant and accurately reproduces the shape and profile of a master part for reproduction in a plastic moulding.
In general, nickel vapour deposition is well known and will be described here only in general terms. U.S. Pat. No. 5,766,683 to Waibel describes a nickel deposition system with a carbon monoxide and vapour recovery system. Pure nickel on exposure to carbon monoxide produces nickel carbonyl vapour, which is contained within an enclosed deposition chamber. The substrate to be coated with a nickel layer is positioned within the chamber and exposed to the nickel carbonyl vapour. When the substrate is heated to a predetermined temperature, the nickel carbonyl vapour decomposes on the substrate. Elemental nickel is plated on the substrate and carbon monoxide gas is emitted. Nickel vapour deposition systems generally include means to withdraw the carbon monoxide gas and recycle the CO gas to produce a continuous supply of nickel carbonyl vapour for deposition.
Depending on the size of the mould or substrate to be coated, nickel vapour deposition generally progresses at a rate much greater than conventional plating to build up a thin plated layer on the substrate at a rate of up to 0.010 inches per hour. During the deposition process, it is critical to maintain the temperature of the substrate within a specified range to ensure that decomposition of the nickel carbonyl gas is maintained and deposition of nickel continues.
Where plastic moulds are produced using nickel vapour deposition, a negative mould surface with a nickel plating layer accurately reproduces the positive master. However, on the opposite surface where deposition occurs, the continuous addition of pure nickel on the surface exposed to the nickel carbonyl vapour rapidly covers surface details, loses definition and a non-uniform layer thickness of nickel results in loss of detail as the thickness of the layer is increased. However, the intent of forming nickel plated moulds is to produce a single side, which is used as an accurate negative of the component to be moulded. The side of the nickel shell opposite the negative mould side is often encased in resins or reinforced in various ways to produce the final nickellined mould. Therefore, the opposite side of the nickel plated layer is given no consideration except to the extent that it provides a base for reinforcing the thin nickel mould surface. The nickel layer is thin and prone to distortion if removed from the master. Therefore mould making includes encasing or reinforcing the deposited side in resins before removal from the master.
It is an object of the present invention to utilize nickel vapour deposition to form sheet metal components having highly complex inner and substantially parallel outer surfaces. The reverse is also true in that the mould could represent the outer surface and the inner deposited surface is parallel to the outer surface.
It is a further object of the invention to provide a means of fabricating sheet metal components that free the designer from the restrictions imposed by conventional manufacturing procedures such as welding and forming from flat plate blanks.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
The invention provides a novel method of manufacturing a seamless self-supporting metal part using nickel vapour deposition, where the part has a selected minimum thickness, a first aerodynamically contoured surface and a second substantially parallel surface. The method replaces labour intensive welding of individually manufactured sheet metal segments for complex aerodynamic shapes, and produces a part with higher accuracy, less cost, controlled strength and hardness properties. Nickel vapour deposition is commonly used for plating electronic components, and for producing a relatively thick metallic lining on plastic injection moulds.
The seamless sheet metal part is manufactured by: fabricating a mould with at least one mould surface being a negative of the first surface of the part; enclosing the mould in a nickel vapour deposition chamber; heating the mould continuously while depositing nickel on the mould surface by nickel vapour deposition until a blank of nickel is deposited having at least the minimum thickness required and an external deposition surface; and separating the blank from the mould.
The invention therefore provides an alternative means to produce complex sheet metal parts. Conventional methods of forming parts from flat plates and welding segments together involves a significant compromise in that the efficient design of sheet metal components is limited by the manufacturing methods that are currently economical.
Cutting flat blanks from flat metal sheets and forming the blanks, assembling the formed segments in a jig and welding involves significant expenditure of time and skilled labour and inevitably results in heat distortion and inaccuracies due to the inherent difficulty in applying heat to thin sheet metal during welding and accurate fitting and assembling.
Nickel vapour deposition as well results in a pure nickel coating, which has limited application due to its relatively low resistance to heat. However, the inventors have recognized that nickel vapour deposition, despite its limitations may provide an economic means for manufacturing specific components which in an aircraft engine are not exposed to high heat such as an exhaust mixer for an aircraft engine.
The invention therefore provides means to manufacture a seamless one-piece sheet metal component which is free from thermally induced distortions and stresses, mechanical stresses from cold working and the inherent increased risk of fatigue failure due to cold working of sheet metal. The invention ensures smooth aerodynamic shapes without the need to grind welds or otherwise finish the external and internal surfaces. Also, the nickel vapour deposition chamber provides a contaminant free environment in which a pure nickel homogenous material may be deposited. The pure nickel component has enhanced strength, hardness and improved durability as a result of manufacture in a seamless manner.